The invention relates to a method for the preparation of a solderable layer system with an electrically conductive fully oxidized layer (conductive layer) on an indium and/or tin base, and a solderable layer (solder layer), where the conductive layer is applied first to a support and then the solder layer is applied to the conductive layer.
Transparent thin-film electrodes usually consist of a material that is not wetted by solders of the usual composition. They must therefore be prepared specially if a usable solder joint is to be made. The technological problems arising in this connection are basically solved. Thus, a method has found acceptance, for instance, in which the conducting layer is first coated with an adhesion-aid layer, then with a layer of solder, and finally, with a protective layer. The adhesion aid takes care of providing a firm hold of the solder layer, and the protective layer prevents corrosion of the solderable material. A typical order of layers in this connection is CrCuAu see German Published Prosecuted Application DE-AS No. 23 33 206 (British Patent Specification No. 1,381,429) or German Published Non-Prosecuted Application DE-OS No. 23 50 000 (U.S. Pat. No. 4,106,860).
Such a layer build-up takes care of a strong low-resistance solder joint but is relatively complicated and expensive. These disadvantages come to bear particularly if as in liquid crystal displays, economic consideration play a central role.
In a simpler coating method, the substrate is first given a partially oxidized electrode which is then coated with copper or iron which in turn is surrounded by a protective layer of a corrosive metal such as silicon, tin or indium; subsequently the electrode is fully oxidized at temperatures above 400.degree. C. in order to make it sufficiently transparent and conductive. The high temperature treatment also has the effect of completely oxidizing the protective layer and the solder layer enters into a strong bond with the electrode by diffusion. In this manner good results are obtained without adhesion aids and noble metals. It is found, however, that the conductive layer does not assume an etching behavior which is easy to control, if there is a pronounced copper or iron diffusion. In addition, a special protective film is still necessary which, however, can fulfill its function reliably only if it is so thick that it no longer allows the solder to get into the layer below and therefore must be etched off.
The manufacture can be made still more efficient if the procedure is as follows: the electrode is coated with a solder layer which already contains a share of corrosive metal and the conductive layer is then converted into its oxides. During the heat treatment required therefore, the corrodable addition oxidizes at the surface of the solder layer, so that a protective skin is formed by itself. This variant affords considerable advantages in production but in practice is not without problems. The oxide-rich outside regions of the solder layer sometimes have too weak a blocking effect, and the solder contact can be stressed mechanically only little in some cases, even if the heavily oxide containing solder layer zones are removed. Apart from that, there is also the danger here that the conductive layer cannot be etched in a defined manner.